Not applicable.
Not applicable.
The invention is related to a catadioptric objective comprising two intermediate images.
Such is known from U.S. Pat. No. 4,701,035 to Hirose as a microlithographic projection exposure system. The objective shown there in FIG. 12 comprises two catoptric partial objectives and one catadioptric partial objective. All objectives are off-axis, not axially symmetric, purely spherical systems.
Catadioptric objectives with one intermediate image and a refractive partial objective are known as microlithographic projection systems with axial symmetry and central 09/263,788) to Schuster, the latter being assigned to the assignee of this invention, and incorporated herein by reference.
Elliott and Shafer show the intermediate image near to the central opening of one of the mirrors, and lenses are arranged in the light path between the mirrors forming Mangin mirrors. All their optical surfaces are spherical.
Schuster shows only the mirrors to be aspherical and avoids big lenses in the beam path between them.
U.S. Pat. No. 5,004,331 to Haseltine et al. discloses a catadioptric projector for projecting an image to a dome (of a flight simulator). The system comprises an external entrance pupil as means for receiving substantially collimated light, a refractive subsystem of rotationally symmetric, coaxial lenses forming a pupil image which is situated at the central opening of an aspheric concave mirror, which together with another concave mirror forms a reflective pupil relay system. Both mirrors are tilted with respect to the optical axis of the refractive subsystem. The whole system provides a wide field of view image on a spherical dome. Full visible spectrum colour correction is obtained by combination of different glass.
It is an object of the invention to provide new design alternatives which allow for high resolution objectives with reduced lens diameters and high performance. Advantageously these designs are to be used in the VUV spectral region for microscopy or microlithography.
The solution of this problem is obtained by an objective comprising axial symmetry, at least one curved mirror and at least one lens and two intermediate images. The objective includes two refractive partial objectives and one catadioptric partial objective. The objective includes a first partial objective, a first intermediate image, a second partial objective, a second intermediate image, and a third partial objective. At least one of said partial objectives is purely refractive. One of the partial objectives is purely refractive and one is purely catoptric.
Axial symmetry together with two intermediate images, two refractive and one catadioptric partial objectives, two intermediate images and at least one refractive partial objective are varied descriptions of the novel aspects of the invention.
Another aspect that clearly groups the mirrors in one catoptric partial objective, which cooperates with one or more purely refractive partial objectives. In this case it is provided that the catoptric partial objective carries the burden of Petzval sum reduction or field flattening. This relieves the refractive partial objective from the need for beam contractions and expansions by negative and positive lens groups, as is long established with microlithographic projection exposure lenses, see e. g. Glatzel E., ZEISS-Information 26 (1981), p. 8-13, U.S. Pat. No. 5,260,832 or U.S. Pat. No. 5,903,400. In consequence the refractive partial objective is simplified and the lens diameters are reduced. Especially for the proposed use in the VUV spectral region this gives great relief to the materials supply of suitable crystals or quartz glasses.
The preferred embodiments also are related to the cited Schuster or Elliott and Shafer designs with two coaxial central obscuration opposing convex mirrors, which allows for a very convenient axial symmetric construction of the objective. Such inter alia has advantages in mechanical rigidity and in compatibility with established stepper/scanner architectures adapted to refractive objectives.
As a central obscuration in principle has degenerating effects in imagingxe2x80x94though in many cases decidedly taken advantage of as in annular or quadrupole illumination or in pupil filtering and apodisationxe2x80x94the reduction of the obscuration by the central hole of the mirrors of this design is of importance.
A preferred way of reducing obscuration is achieved by placing the intermediate images in the vicinity of the mirrors.
In an alternative embodiment, lenses are inserted between the mirrors. As negative lenses these cooperate with the mirrors to give single material colour correction, relieving the need for band narrowing the laser light source or for using an achromatizing material pair in the VUV.
The chief ray height at each of the mirror bores is approximately the same in value, but opposite in sign. This measure allows for minimal central obscuration.
The sequence where the mirror-containing partial objective is framed by the two refractive partial objectives is preferred as it allows for both intermediate image xe2x80x9cplanesxe2x80x9d connected by the mirror containing partial objective to be curved such as to best exploit the specific correction capabilities of this partial objective.
While it is rather conventional that mirrors are aspheric also in the related art, in the present invention it is specifically stated that aspheric lens surfaces prove advantageous with this design. All advantages and restrictions as recently established for refractive projection exposure objectives, see e. g. patent application DE 199 22 209 of Schuster and references cited therein, as incorporated herein by reference, hold also for the use of aspheric surfaces in the designs of this invention.
Diffractive surfaces, as occasionally also proposed for projection exposure objectives, are also useful with this invention just as they are with refractive designs.